This section describes the facilities that PostgreSQL's libpq client interface library provides for
accessing large objects. The PostgreSQL large object interface is modeled
after the Unix file-system
interface, with analogues of open,
read, write, lseek,
etc.

All large object manipulation using these functions
must take place within
an SQL transaction block, since large object file descriptors are
only valid for the duration of a transaction.

If an error occurs while executing any one of these functions,
the function will return an otherwise-impossible value, typically
0 or -1. A message describing the error is stored in the
connection object and can be retrieved with PQerrorMessage.

Client applications that use these functions should include
the header file libpq/libpq-fs.h and
link with the libpq library.

creates a new large object. The return value is the OID that
was assigned to the new large object, or InvalidOid (zero) on failure. mode is unused and ignored as of
PostgreSQL 8.1; however, for
backward compatibility with earlier releases it is best to set
it to INV_READ, INV_WRITE, or INV_READ|INV_WRITE.
(These symbolic constants are defined in the header file
libpq/libpq-fs.h.)

An example:

inv_oid = lo_creat(conn, INV_READ|INV_WRITE);

The function

Oid lo_create(PGconn *conn, Oid lobjId);

also creates a new large object. The OID to be assigned can
be specified by lobjId; if so,
failure occurs if that OID is already in use for some large
object. If lobjId is InvalidOid (zero) then lo_create assigns an unused OID (this is the
same behavior as lo_creat). The
return value is the OID that was assigned to the new large
object, or InvalidOid (zero) on
failure.

lo_create is new as of
PostgreSQL 8.1; if this
function is run against an older server version, it will fail
and return InvalidOid.

filename specifies the
operating system name of the file to be imported as a large
object. The return value is the OID that was assigned to the
new large object, or InvalidOid (zero)
on failure. Note that the file is read by the client interface
library, not by the server; so it must exist in the client file
system and be readable by the client application.

also imports a new large object. The OID to be assigned can
be specified by lobjId; if so,
failure occurs if that OID is already in use for some large
object. If lobjId is InvalidOid (zero) then lo_import_with_oid assigns an unused OID
(this is the same behavior as lo_import). The return value is the OID that
was assigned to the new large object, or InvalidOid (zero) on failure.

lo_import_with_oid is new as
of PostgreSQL 8.4 and uses
lo_create internally which is new
in 8.1; if this function is run against 8.0 or before, it will
fail and return InvalidOid.

The lobjId argument specifies the
OID of the large object to export and the filename argument specifies the operating
system name of the file. Note that the file is written by the
client interface library, not by the server. Returns 1 on
success, -1 on failure.

The lobjId argument specifies the
OID of the large object to open. The mode bits control whether the object is opened
for reading (INV_READ), writing
(INV_WRITE), or both. (These symbolic
constants are defined in the header file libpq/libpq-fs.h.) lo_open returns a (non-negative) large object
descriptor for later use in lo_read, lo_write, lo_lseek, lo_lseek64, lo_tell, lo_tell64, lo_truncate, lo_truncate64, and lo_close. The descriptor is only valid for
the duration of the current transaction. On failure, -1 is
returned.

The server currently does not distinguish between modes
INV_WRITE and INV_READ|INV_WRITE: you are allowed to read from the
descriptor in either case. However there is a significant
difference between these modes and INV_READ alone: with INV_READ you cannot write on the descriptor, and
the data read from it will reflect the contents of the large
object at the time of the transaction snapshot that was active
when lo_open was executed,
regardless of later writes by this or other transactions.
Reading from a descriptor opened with INV_WRITE returns data that reflects all writes
of other committed transactions as well as writes of the
current transaction. This is similar to the behavior of
REPEATABLE READ versus READ COMMITTED transaction modes for ordinary
SQL SELECT commands.

writes len bytes from buf (which must be of size len) to large object descriptor fd. The fd argument
must have been returned by a previous lo_open. The number of bytes actually written
is returned (in the current implementation, this will always
equal len unless there is an error).
In the event of an error, the return value is -1.

Although the len parameter is
declared as size_t, this function will
reject length values larger than INT_MAX. In practice, it's best to transfer data
in chunks of at most a few megabytes anyway.

reads up to len bytes from large
object descriptor fd into buf (which must be of size len). The fd
argument must have been returned by a previous lo_open. The number of bytes actually read is
returned; this will be less than len
if the end of the large object is reached first. In the event
of an error, the return value is -1.

Although the len parameter is
declared as size_t, this function will
reject length values larger than INT_MAX. In practice, it's best to transfer data
in chunks of at most a few megabytes anyway.

To change the current read or write location associated with
a large object descriptor, call

int lo_lseek(PGconn *conn, int fd, int offset, int whence);

This function moves the current location pointer for the
large object descriptor identified by fd to the new location specified by offset. The valid values for whence are SEEK_SET
(seek from object start), SEEK_CUR
(seek from current position), and SEEK_END (seek from object end). The return value
is the new location pointer, or -1 on error.

When dealing with large objects that might exceed 2GB in
size, instead use

This function has the same behavior as lo_lseek, but it can accept an offset larger than 2GB and/or deliver a result
larger than 2GB. Note that lo_lseek will fail if the new location
pointer would be greater than 2GB.

lo_lseek64 is new as of
PostgreSQL 9.3. If this
function is run against an older server version, it will fail
and return -1.

This function truncates the large object descriptor
fd to length len. The fd
argument must have been returned by a previous lo_open. If len is
greater than the large object's current length, the large
object is extended to the specified length with null bytes
('\0'). On success, lo_truncate
returns zero. On error, the return value is -1.

The read/write location associated with the descriptor
fd is not changed.

Although the len parameter is
declared as size_t, lo_truncate will reject length values larger
than INT_MAX.

When dealing with large objects that might exceed 2GB in
size, instead use

int lo_truncate64(PGcon *conn, int fd, pg_int64 len);

This function has the same behavior as lo_truncate, but it can accept a len value exceeding 2GB.

lo_truncate is new as of
PostgreSQL 8.3; if this
function is run against an older server version, it will fail
and return -1.

lo_truncate64 is new as of
PostgreSQL 9.3; if this
function is run against an older server version, it will fail
and return -1.